As a well is drilled, sections of casing are positioned in the wellbore to contain formation fluid within the formation and prevent such fluid from entering into the wellbore. In order to secure the casing in position, a cement slurry is pumped down the interior of the casing and then is forced up through the annulus between the casing and the wellbore. After solidifying, the cement locks the casing into the wellbore.
In order to prevent the cement slurry from returning to the interior of the casing, downhole cementing equipment is used that include check valves. A shoe is placed at the bottom of the casing string. The shoe may be either a float shoe which has a check valve or a guide shoe which does not. A float collar having a check valve is placed one to five sections of casing above the float shoe or guide shoe. Cement is used in the manufacture of the cementing equipment, as it is easily drillable. The cement used in the manufacture is similar in composition to the cement slurry that is pumped into the well to retain the casing string within the wellbore. Cementing plugs are used to separate the well fluid from the cement. A bottom plug is placed into the casing before cement is pumped into the casing. A top plug is placed on top of the cement after it has been completely pumped into the casing. During the pumping process, both of these plugs are displaced down on top of the float collar.
After the cementing is completed and the cement has solidified, the well operator will re-enter the casing with a drill bit to drill the well to a greater depth. In doing so, the operator will drill through the cementing plugs, the float collar, the cement remaining inside the casing between the float collar and the shoe, until the undrilled formation is reached.
At the present time it is very difficult to determine the location of the drill bit while drilling out the cementing equipment from a wellbore. This is most important if the cement plugs have not been completely displaced to the top of the float collar. It is not uncommon for this to occur if incorrect calculations have been made for displacement of fluid, the method of determine displacement of fluid volume is incorrect, or if the check valves in the cementing equipment did not operate properly.